A code of conduct for supermarkets comes into effect on Sunday in an effort to ensure a fairer deal for farmers - but many fear it does not go far enough.

The code does not cover milk, meat or bread processing.

And smaller producers will still be vulnerable to the buying power of the supermarkets, according to vice-president of the National Farmers' Union Michael Paske, an asparagus grower from Lincolnshire

He told BBC News: "It is very difficult for a small producer to go along and say, 'I do not think you are being fair to me'."

Price cut

Organic dairy farmers say they are being driven out of business by Tesco cutting the cost of organic milk by up to 2.5p a litre.

The supermarket says it has not dropped the price it pays to farmers and that its price cut will encourage sales and reduce the surplus of milk on the market.

But farmers are worried other supermarkets will have to compete and put pressure on their suppliers to deliver milk at a lower price.

This would cause the knock on effect that goes with a supermarket price war.

BBC correspondent Virginia Eastman says the new code of conduct is designed to iron out such disputes.

Critics of the code say it has no teeth because it does not include the crucial link in the chain - the food processors.

Mediation

The dispute between supermarkets and farmers looks set to be its first test.

Tesco, Asda, Sainsbury and Safeway have all agreed to comply with the code, drawn up by the Director General of Fair Trading.

It seeks to ensure that their terms are available in writing and transparent, and that the supermarkets will have to give advance notice of changes or compensate their suppliers.

Complaints about possible breaches can be taken to an independent mediator. The cost of mediation in each case will be borne by the supermarket concerned.

The Director General can investigate cases if mediation fails.